The present invention relates to the so-called turn-of-nut tightening method of controlling the tightening of a screw-turning device, depending upon the rotational angle.
Screw tightening devices may be classified according to two different types.
The first type employs a calibrated torque wrench for tightening to a specified torque.
The second type of control device employs the rotational angle control, termed "turn-of-nut" tightening type wherein the tightening of the screw is managed in accordance with the rotational angle of the screw during tightening. The turn-of-nut tightening method enables the management of the tightening force to a high degree of accuracy compared with the calibrated torque wrench tightening type of device. This is because the tightening force, that is the axial tension of the screw, increases according to the angle of tightening the screw. Japanese Patent Laid Open No. 58-56776 employs a tightening method commonly referred to as the snug torque method. A more accurate tightening method is employed in the turn-of-nut tightening method as disclosed in Japanese Patent Laid Open No. 62-246482, wherein a linear portion of the torques vs. rotational angle characteristic curve during tightening of the screw is found and extended to where the torque was zero in accordance with a linear expression to determine a tightening starting point at the intersection.